Physical simulation on oil charging process and controlling factor in reservoirs of Wenchang formation and Enping formation in Zhu-I Depression, Pearl River Mouth Basin

    The Wenchang Formation and Enping Formation in the Pearl River Mouth Basin have huge oil and gas potential, but the migration and accumulation characteristics are not clear, which seriously restricts the large-scale exploration and development of oil and gas.In combination with thin section,scanning electron microscope and high-pressure mercury injection, physical modeling experiments of oil charging were conducted to find out laws and affecting factors of oil migration and seepage in reservoirs using core samples from reservoir beds of the Wenchang formation and Enping formation in Zhu-I Depression, Pearl River Mouth Basin. The growth curve of oil saturation presents three stages: rapid growth, slow growth and stabilization, and the final oil saturation ranges from 30% to 80%. Reservoir pore types are mainly intergranular pore, dissolution pore and fracture, and reservoir can be divided into three types: high porosity-high permeability, high porosity-low permeability and low porosity-low permeability. At the same time,The growth modes of oil saturation can also be divided into three types: Type Ⅰ is rapid speed growth-high saturation type, corresponding to high porosity-high permeability reservoir; The Type Ⅱ is medium speed growth- medium saturation type, corresponding to high porosity-low permeability reservoirs. The Type Ⅲ is slow speed growth-low saturation type, corresponding to low porosity-low permeability reservoir. The microscopic model diagram of oil charging shows that with the change of reservoir type from high porosity-high permeability to low porosity-low permeability, the main pore types of charging change from intergranular pore and dissolution pore to dissolution pore and fracture, and the growth mode of oil saturation also changes from type I to type III. The accumulation process and flow characteristics of crude oil are dominantly influenced by the injection pressure and pore structure of reservoirs.The injection pressure is a prerequisite for the increase of oil saturation,and pore structure is the main factor to control the growth pattern of oil saturation. Based on the experimental results,the relationship diagram of porosity-permeability- charging pressure-oil saturation is established. According to the distribution of residual pressure and the relationship diagram,the lower porosity-permeability limit of the reservoir to reach the specific oil saturation under different residual pressure can be determined. This is conducive to the dynamic analysis of oil and gas charging process and the prediction of oil saturation under different physical properties and dynamic conditions.